In variable rate vocoders systems, such as IS-96, IS-127 (EVRC), and CDG-27, there remains the problem of distinguishing between voice and background noise in moderate to low signal-to-noise ratio (SNR) environments. The problem is that if the Rate Determination Algorithm (RDA) is too sensitive, the average data rate will be too high since much of the background noise will be coded at Rate 1/2 or Rate 1. This will result in a loss of capacity in code division multiple access (CDMA) systems. Conversely, if the RDA is set too "lean", low level speech signals will remain buried in moderate levels of noise and coded at Rate 1/8. This will result in degraded speech quality due to lower intelligibility.
Although the RDA's in the EVRC and CDG-27 have been improved since IS-96, recent testing by the CDMA Development Group (CDG) has indicated that there is still a problem in car noise environments where the SNR is 10 dB or less. This level of SNR may seem extreme, but in hands-free mobile situations this should be considered a nominal level. Fixed-rate vocoders in time division multiple access (TDMA) mobile units can also be faced with similar problems when using discontinuous transmission (DTX) to prolong battery life. In this scenario, a Voice Activity Detector (VAD) determines whether or not the transmit power amplifier is activated, so the tradeoff becomes voice quality versus battery life.
Thus, a need exists for an improved apparatus and method for rate determination in communication systems.